This invention relates to a practical, low cost system which operates on demand to initiate and to terminate a defrost cycle for refrigeration and heat pump systems, and more particularly to an improved system which uses a capacitive sensor to detect frost build up.
There have been proposals in the prior art for systems using capacitive sensors to sense the build up of frost on surfaces of a refrigeration or heat pump unit in order to initiate a defrost cycle. U.S. Pat. No. 2,904,968, issued Sept. 22, 1959, to J. A. Spencer, Jr., is an example of such prior art system. Although fundamentally sound, such an approach has not begn susceptable to practical low cost implementation, and has not found heretofore widespread acceptance.
To detect frost build up, a conductive plate is placed adjacent to the surface of the evaporator. The capacitive reactance between the plate and the surface varies as frost builds up on the surface. It will be appreciated that the conductive plate must be separated from the surface a distance which is sufficient to insure that no water in a liquid phase will be held between the plate and the surface due to surface tension.
An object of this invention is the provision of a capacitance demand defrost system which is simple, reliable in operation, and which can be readily implemented by means of integrated circuitry in order to realize low cost.
Briefly, this invention contemplates the provision of a demand defrost system in which a epoxy resin hermetically seals the conductive plate in order to minimize the leakage resistance between the plate and the surface. A noise immune phase detector detects a phase shift caused by the build up frost; when the build up exceeds a predetermined amount a defrost cycle commences, and when it recedes the cycle is automatically terminated .